diff --git a/vchan/io.c b/vchan/io.c
index 5d32b4e9..d6b7ac92 100644
--- a/vchan/io.c
+++ b/vchan/io.c
@@ -129,7 +129,7 @@ int libvchan_wait(struct libvchan *ctrl)
         may write less data than requested;
         returns the amount of data processed, -1 on error or peer close
 */        
-int libvchan_write(struct libvchan *ctrl, char *data, int size)
+int libvchan_write(struct libvchan *ctrl, const char *data, int size)
 {
 	int avail, avail_contig;
 	int real_idx;
diff --git a/vchan/libvchan.h b/vchan/libvchan.h
index 6b19b2fa..4307346d 100644
--- a/vchan/libvchan.h
+++ b/vchan/libvchan.h
@@ -78,7 +78,7 @@ struct libvchan *libvchan_server_init(int devno);
 struct libvchan *libvchan_client_init(int domain, int devno);
 
 int libvchan_server_handle_connected(struct libvchan *ctrl);
-int libvchan_write(struct libvchan *ctrl, char *data, int size);
+int libvchan_write(struct libvchan *ctrl, const char *data, int size);
 int libvchan_read(struct libvchan *ctrl, char *data, int size);
 int libvchan_wait(struct libvchan *ctrl);
 int libvchan_close(struct libvchan *ctrl);